PHOTOCHEMISTRY OF O-VINYLBENZALDEHYDE : FORMATION OF A KETENE METHIDE INTERMEDIATE AND ITS TRAPPING WITH SECONDARY AMINES

Irradiation of o-vinylbenzaldehyde (1) in the presence of primary and secondary amines results in N−H addition across the two chromophores of 1, to give o-ethylbenzamides 6. Photoreaction of deuterium-labeled aldehyde 15 with piperidine gave an amide (17) carrying a deuterium at the β-carbon of the ethyl side chain whereas use of N-deuteriopiperidine led to deuterium incorporation at the α-carbon (14). These results are explained on the basis of a 1,5 hydrogen shift in the excited state of 1 to give a ketene methide intermediate (7) which becomes trapped with amines. Upon 308 nm laser excitation of 1 in acetonitrile or benzene solution, a weak transient absorption having λmax at 380 nm was observed. Irradiation of 1 isolated in an argon matrix with 313 nm light also revealed formation of an intermediate with a UV absorption maximum around 380 nm. Its IR spectrum displayed characteristic ketene stretching vibrations at 2086 and 2098 cm-1, providing definitive support for the ketene methide structure 7.